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Author Topic: Question about skip  (Read 2656 times)
KF5PGT
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« on: April 06, 2013, 08:22:25 AM »

I have a basic understanding (I think) of how radio propagation works.

Earlier today I heard half of a conversation on 10 meters between a guy in California and a guy just about 75ish miles fom me. I could hear the Cali ham but not the ham closest to me. From what I understand, this is because of skip. Either that or the ham local to me was using a yagi or something else directional.

But here's where I get confused. There's a local ham club that holds a 10 meter net. They hold it at a time that I'm usually at work but I've tried to tune in once or twice. I hear nothing. They're about 25-30 miles from me. I can hit their 2 meter repeater with my HT in my backyard. How is it that the other hams in the club can check into that net but I can't hear any of them?

From what I understand, antenna polarization is not much of an issue on HF. I have a homebrew dipole which was either horizontal or inverted vee the last time I tried to catch the net. It's a inverted vee now and I've made contacts in the US, Carribean, and Europe.

Would having a vertical antenna get me into the net?
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AA4PB
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« Reply #1 on: April 06, 2013, 08:35:56 AM »

From your station out to some local distance you will receive a direct wave. Then there will be a large skip zone where the signal is overhead. Then it comes back to the ground at some greater distance so you hear the distant stations, but not those in the skip zone.

Antenna polarization is not important when receiving via skip because the ionosphere can rotate the polarization of the signal so you never know what you are going to get. Polarization is quite important when receiving the local direct signal. Vertical or horizontal isn't important but it is important that the transmit and receive stations use the same polaritiy. Vertical to horizontal cross-polarization can add as much as 20dB to the signal path loss.

Whether or not vertical polarization would help you with the local net depends mostly on what the other stations are using. Also note that a horizontal close to ground (less than 1/2 wavelength) will have a high angle of radiation which is not what you want for direct wave. The horizontal can also be directional and the network stations may be located off the ends.
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AA4PB
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« Reply #2 on: April 06, 2013, 08:49:40 AM »

Here's a real-life experience that I have on 6M. I have a fairly good 6M station with a 5 element Yagi at 65 feet. There is a weekly net that takes place some distance south of me. The stations on the north side of that net I can work any time with their Yagis pointed north. During the net however they point south in order to hear most of the net stations so I am off the back of their antennas so I only hear them down in the noise. The stations south of the primary net coverage area have their antennas pointed north, towards me but they are too far away to get easily copyable signals.
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KF5PGT
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« Reply #3 on: April 06, 2013, 09:00:39 AM »

Isn't an inverted vee dipole supposed to be omnidirectional? Maybe I had it horizontal at that time. I don't remember if I switched it to a vee before or after I last tried to tune in. Either way, the center of the vee is a little over half wave above ground (about 16'-17').

I guess I'll have to rig up a vertical for next time.
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AA4PB
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« Reply #4 on: April 06, 2013, 09:24:28 AM »

No, an inverted-V is pretty much the same as a flat-top dipole. The maximum radiation is off the sides. Much depends on what the other stations are using for antennas. Vertical, horizontal, wire dipoles, Yagis, etc. I'd say that 25-30 miles direct wave between two stations using fairly low wire dipoles is on the edge. The reason you can work the area repeater with an HT in the back yard is because that repeater antenna is probably at least several hundred feet up in the air. If their 10M antenna were in that same location you'd hear it loud and clear too.

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KF5PGT
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« Reply #5 on: April 06, 2013, 09:43:07 AM »

That's a good point about the repeater antenna. I'm gonna have to get in touch with some of the guys that get into that net and pick their brains about their antenna setups.
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M6GOM
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« Reply #6 on: April 06, 2013, 11:45:18 AM »

Here is how it all works on HF.

http://www.tpub.com/neets/book10/NTX2-19.GIF

Ground wave is how far the signal travels along the ground. You then have the Sky Wave Coverage which is where the signals bounced off the ionosphere land and inbetween the ground wave coverage and the sky wave coverage is the skip zone where you will not be able to hear or reach any stations.

They may be able to have the 10m net because they're all within the ground wave region and you aren't. Ground wave on 10m is only a few dozen miles - 50 at the best.
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KE3WD
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« Reply #7 on: April 06, 2013, 12:13:40 PM »

Isn't an inverted vee dipole supposed to be omnidirectional? Maybe I had it horizontal at that time. I don't remember if I switched it to a vee before or after I last tried to tune in. Either way, the center of the vee is a little over half wave above ground (about 16'-17').

I guess I'll have to rig up a vertical for next time.

Sounds like you may be confusing omnidirectional with polarity.  Two different things. 


73
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K8AXW
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« Reply #8 on: April 06, 2013, 12:17:30 PM »

PGT:  And you thought you was going to ask a simple question!   Roll Eyes   Grin

The drawing at the link GOM provided, while good, is incomplete.  There is the ground wave component, the direct wave and then the skip or ionosphere reflection.

The ground wave signal actually follows the ground and this is why the frequencies used by AM broadcast stations are between 500KHz and 1500KHz.

The direct wave is the signal that is transmitted and received if the transmit and receive antennas can "see" each other.

The sky wave is the signal that is reflected from the ionosphere.  It isn't uncommon for this signal to "bounce" back and forth from the ionosphere to ground several times providing "multiple hop" communications.

The answer to your question becomes even more involved when you consider the ionosphere  is actually in layers and changes with the time of day. The effects change along with it.

Then there is the relationship between the ionosphere and frequency.  Some frequencies are never affected by the ionosphere and some frequencies "punch" through the ionosphere and continue out into space.

This information is readily available from many publications, including those provided by the ARRL.  I'm sure you can also "Google" this and find enough to keep you busy for quite some time!  It is a very interesting subject.
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KF5PGT
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« Reply #9 on: April 06, 2013, 08:16:55 PM »

I thought that vertically polorized antennas were omni directional?

If the ground wave on 10 meters is around 25-50 miles, I should be able to pick up at least one of the guys in that net. Most of them are between 20 to 30 miles away from me. I'd think we'd be on the long end of that estimate down here in south Louisiana. Pretty much flat land.

Radio propagation is a weird yet fascinating subject. Right now I only have 2 and 10 meter capabilities, but I hoping to get into a bigger, better radio very soon. I have fairly limited time each day to play radio so I'm looking to get on 20 and 40 meters to maximize my play time!
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K8AXW
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« Reply #10 on: April 06, 2013, 09:13:42 PM »

Normally vertical antennas are omnidirectional.  However you can also have vertically polarized beams or phased antennas.  (more than one antenna being used at the same time)

It's been my experience that 25-50 miles on 10m would be a stretch.  However, with flat terrain, the direct wave might be 25 or more miles..... but again, I think 50 miles would again be a stretch.

You're best bet would be to ask your friends what they are doing.  Oh, you are trying to hear them on USB, right? Then again, they might be using LSB to throw crap into the game!
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KQ6Q
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« Reply #11 on: April 07, 2013, 02:32:27 AM »

PGT - the fact that you can hit the 2 meter repeater of the club with the 10 meter net isn't relevant - it's on a tower or tall building or mountain peak. Unless they're using a 10 meter FM repeaters, they're working from their home stations, with horizontal or vertical antennas. If they're using vertical antennas, and you're using vertical antenna too, you'll either hear them on groundwave, or not. If they're using horizontal antennas, you might hear them, but likely not. polarization matters. When I was a new novice, my elmer buddy lived about 20 miles away, used a vertical on 40 meters. I had a dipole on 40 meters, and he was off the end. We could NOT hear each other at all. Just the way it was on 40. We could have worked on 2m if we could have seen the same repeater.
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AD6KA
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« Reply #12 on: April 07, 2013, 12:09:36 PM »

Quote
I could hear the Cali ham but not the ham closest to me.

From what I understand, this is because of skip.

I know a guy named Skip! He lives in Woodland Hills
and installs towers and stuff.

"Skip" is a CB term and probably best avoided
in conversations on the ham bands. Personally I
don't care, <shrug> But some oldtimers/grouches/purists
 may give you a hard time.
Just an FYI so you are forewarned.....
GL ES HAVE FUN!
73, Ken  AD6KA
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NO2A
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Posts: 768




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« Reply #13 on: April 07, 2013, 12:18:19 PM »

Even on a band like 40m,you may find it harder to hear states nearby to you late at night when 40 really opens up. For me in NJ,I might be able to hear PA during the day on 40. At night would be much more difficult,maybe not impossible but they would be weak. 80m is the better band for very local work. Or 2m.
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AA4PB
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Posts: 12770




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« Reply #14 on: April 07, 2013, 01:27:42 PM »

"Skip" is a CB term and probably best avoided
in conversations on the ham bands.

Come on now. "Skip" is a technical term that you'll find in most documentation on propogation and it was used by hams long before there was any such thing as 11M CB. I think the OTs are giving CB too much credit. Many of the terms used on CB originated with ham radio.

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